1. Field of the Invention
The present invention relates to an illumination device comprising a point light source that diffuses and emits light emitted from the point light source, and to a display device comprising this illumination device.
2. Description of the Related Art
In recent years, the use of LEDs (light emitting diodes) as illumination devices for in-home use and light sources for backlights of liquid crystal display devices in place of fluorescent lamps and cold-cathode fluorescent lamps (CCFL) has been actively examined. LEDs are more environmentally friendly when compared with CCFLs and the like, and have the potential to reduce energy consumption in the future. However, LEDs are small when compared with illumination devices, and resemble point light sources. Therefore, the light emitted from an LED must be linearly or in a plane diffused. Additionally, the amount of light emitted per LED is smaller than the amount of light emitted per fluorescent lamp or cold-cathode fluorescent lamp. Several LEDs must therefore be used in order to produce the same amount of light as a fluorescent lamp or cold-cathode fluorescent lamp.
However, characteristics of commercially available LEDs such as the amount of light emitted vary greatly in individual units, and, for example, a difference of about ±15% in luminosity may occur. For this reason, when a plurality of LEDS are used in the manufacture of an illumination device, irregularities in illumination intensity and brightness occur. These irregularities in illumination intensity and brightness are a serious problem particularly when such an illumination device is used as the backlight of a liquid crystal display device. To solve this problem, the LEDs mounted in the illumination device must be adequately sorted. However, when the LEDs are sorted, the cost of the device increases. Therefore, when a plurality of LEDs are provided as the light sources of an illumination device, light emitted by the LEDs must be mixed so as to be homogeneous.
Individual LEDs emitting red (R), green (G), and blue (B) light are provided to a single illumination device, and the light emitted from the colored LEDs is mixed to produce white light, thereby making it possible to produce an illumination device having excellent color reproducibility. In such devices as well, the light emitted from the colored LEDS must be mixed so as to be homogeneous.
Publicly known document 1 (Japanese Laid-Open Patent Application 2001-57106 (pages 2-5, FIG. 2)) discloses a technique wherein a plurality of point light sources are provided to an illumination device, a single light-guide body for one-dimensionally diffusing the light emitted from these point light sources is provided to each point light source, and a single transparent substrate for diffusing in a plane the linear light incident on the side surface via the light-guide bodies is further provided. The light-guide body is composed of a transparent material, has a linearly extending wedge shape, and is disposed alongside the transparent substrate. In publicly known document 1, the light emitted from the point light source is thereby linearly diffused by the light-guide body, and further diffused in a plane by the transparent substrate.
Publicly known document 2 (Japanese Laid-Open Patent Application 2001-23423 (pages 2-5, FIGS. 1, 7)) and Publicly known document 3 (Japanese Laid-Open Patent Application 2002-250820 (pages 2, 4-5, FIG. 1)) disclose a technique wherein a single point light source (LED) and a single light-guide member in the form of a plate, onto whose side surface the light emitted from the point light source is incident, are provided to an illumination device, and a blind hole for laterally dividing the light incident from the point light source is formed on the light-guide member. In publicly known documents 2 and 3, the light incident from the point light source on the light-guide member is changed to a planar state and made uniform in a single step by the blind hole and a peripheral reflective surface. In addition, FIG. 7 in publicly known document 2 discloses a technique wherein two point light sources are provided, and two blind holes are formed on the light-guide member.
In addition, publicly known document 4 (Japanese Laid-Open Patent Application 2002-352611 (pages 2-7, FIGS. 1, 2)) discloses an illumination device comprising a plurality of point light sources and a plurality of Fresnel lenses corresponding to the point light sources. The light incident from the point light source on the Fresnel lenses is oriented in a prescribed direction. In addition, for a given area of the Fresnel lenses, the light incident from the point light source corresponding to the Fresnel lens adjacent to this region is also oriented in the prescribed direction when viewed from the optical axis of the Fresnel lens. In publicly known document 4, an illumination device having a uniform distribution of emitted light intensity can thereby be realized.
On the other hand, the present inventors have developed a technique wherein a light source and light-guide plate are provided to an illumination device, the light-emitting surface of the light-guide plate is provided with a light divider for dividing in mutually different directions the light from the light source that is directed into the light-guide plate, and a prism for reflecting the light divided by the light divider toward the light-emitting surface is provided to a region in which light from the light source in not received directly on the light-receiving surface of the light-guide plate. This technique is disclosed in publicly known document 5 (Japanese Laid-Open Patent Application 2005-228700 (pages 2-7, FIG. 1)). The distribution of the quantity of light emitted by this illumination device is uniform, and the angle of the light emitted is narrow.
However, problems such as those described hereafter are presented in the above-described prior art. In the illumination device disclosed in publicly known document 1, a wedge-shaped light-guide body is provided for each point light source. Therefore, a plurality of light-guide bodies are needed when a plurality of point light sources are to be provided, inevitably increasing the cost. Also, the light-guide body is disposed alongside the transparent substrate, and, as a result, the peripheral space inevitably expands when the number of light-guide bodies is increased.
In the illumination devices disclosed in publicly known documents 2 and 3, there is only one position in which the light sources should be disposed with respect to a single blind hole/reflective plate combination, and it is therefore difficult to place a plurality of point light sources in this position. In addition, in FIG. 7 of publicly known document 2, two blind hole/reflective plate combinations are provided, and two point light sources are provided so as to correspond with the combinations. However, in this configuration, the light emitted from the two point light sources cannot be mixed. Therefore, when LEDs are used as the point light sources, irregularities caused by differences between the individual LEDs readily occur. Additionally, because the light emitted from the point light sources cannot be mixed, RGB-emitting LEDs cannot be used as the light source.
Furthermore, in a given region (e.g., a region on the left side) of the Fresnel lenses of the illumination device disclosed in publicly known document 4, light that is incident from the point light source (e.g., a central point light source) corresponding to the Fresnel lens, and light that is incident from another point light source (e.g., a right-side point light source) corresponding to the Fresnel lens adjacent to this region are emitted in the same direction, as seen from the optical axis of the Fresnel lens. Therefore, the light emitted from the two mutually adjacent point light sources can be partially mixed. However, when light emitted from a point light source (i.e., the left-side point light source) corresponding to a Fresnel lens that is adjacent to the opposite side of this region, as seen along the optical axis of the Fresnel lens, is incident on the abovementioned region of the Fresnel lenses, the light cannot be directed in a prescribed direction. Also, even the light incident from point light sources disposed in positions further away cannot be directed in a prescribed direction. For this reason, there is a limit to the light mixing effect, and, while light emitted from, e.g., two mutually adjacent point light sources can be partially mixed, light emitted from three point light sources cannot be uniformly mixed. Consequently, white light is difficult to produce when using RGB-emitting LEDs as the light sources. Additionally, as shown in FIG. 14 of publicly known document 4, only an intensity of about 70% of intensity along the optical axis can be achieved in the peripheral areas of the unit Fresnel lenses, and uniformity is inadequate.
Still further, in the illumination device disclosed in publicly known document 5, the light-guide plate merely divides and emits light emitted from a corresponding light source, and the mixing and homogenization of light emitted from a plurality of light sources is not taken into account.